under GXT 1.2.4 I tried to use SimpleComboBox, but the following error occured:

Caused by: com.google.gwt.user.client.rpc.SerializationException: Type 'com.extjs.gxt.ui.client.widget.form.SimpleComboValue' was not included in the set of types which can be deserialized by this SerializationPolicy or its Class object could not be loaded. For security purposes, this type will not be deserialized.